ENH: add argList support for non-mandatory arguments

- in rare cases we may wish to have command-line arguments that are
  non-mandatory. This can now be reflected in the usage output, provided
  that the argList::nonMandatoryArgs() has been used.

- added setRootCaseNonMandatoryArgs.H that applies the
  argList::nonMandatoryArgs() settings and otherwise performs largely
  as per setRootCase.H, except that the check for mandatory arguments
  is deferred to later user code.